Field of the Invention
The present invention relates to a holographic memory device that records information as holograms in optical information recording media and/or reproduces information from optical information recording media.
Description of the Related Art
As a background art of the technological field, there is US 2014-0043952 (Patent Document 1). This document discloses use of a wedge prism having a reflection film rotationally controlled by a galvano motor and having a tilted optical surface for optimization of a luminous flux diameter of a reference beam.
Further, there is JP-A-2006-78942 (Patent Document 2). This document discloses that “a diffraction grating 111 and a hologram recording material 14 are provided in an imaging relationship with each other via a telecentric optical system including lenses 12, 13, and thereby, even when the angle of incidence of a reference beam 200 is changed by the diffraction grating 111 and the angle of incidence to the hologram recording material 14 is changed, the exposed area of the hologram recording material 14 exposed to the reference beam 200 may be made constant because the diffraction grating 111 and the hologram recording material 14 have the imaging relationship. In addition, this effect may be realized without damage on the fast scan speed and the small and light configuration or without mechanical wear.
A holographic memory system causes interference between a signal beam and a reference beam and records its interference pattern as a hologram in an optical information recording medium. For example, in the double-luminous flux angular multiplexing system, multiplexing recording of holograms is performed while changing the angle of incidence of the reference beam in the same position within the optical information recording medium. Further, at reproduction, the reference beam is allowed to enter the optical information recording medium at the same angle of incidence like that at recording, a recovered beam diffracted from the holograms is detected using an image sensor, and thereby, the information recorded in the optical information recording medium is reproduced.
At recording/reproduction of the holograms, when the optical information recording medium is exposed to light that does not contribute to recording/reproduction (hereinafter, referred to as “stray light”), production of unnecessary exposed regions and formation/reproduction of unintended holograms are caused. By the production of unnecessary exposed regions, M# (M-number) as an index indicating the multiplexing recording performance of the optical information recording medium is wasted, and problems arise in high-capacity high-density recording. Further, by the formation/reproduction of unintended holograms, the diffracted beam and the recovered beam from the holograms interfere on the image sensor and cause deterioration in quality of reproduced signals. As described above, when stray light is generated at holographic recording/reproduction, stable signal reproduction becomes harder.
In example 6 of Patent Document 1, for optimization of the luminous flux diameter of the reference beam, the wedge prism having the reflection film rotationally controlled by the galvano motor and having the tilted optical surface is used. In this case, a surface-reflected beam of the wedge prism becomes stray light, however, Patent Document 1 does not disclose the problem.
Further, in Patent Document 2, the diffraction grating is used for optimization of the luminous flux diameter of the reference beam, and actually, not only a desired diffracted beam of the diffraction grating but also a zero-order beam and a high-order diffracted beam appear and the beams become stray light. However, Patent Document 2 does not disclose the problem.